Gas-filled radiation detectors have been used for many years to provide qualitative and quantitative information concerning nuclear radiation. Such a detector consists of a hollow cathode defining a gas-filled chamber, and an anode within the chamber electrically insulated from the cathode. A voltage is applied between the anode and cathode. When the detector is placed in a radiation field, nuclear particles enter the chamber, causing ionization and the release of electrons. The ions and electrons are collected and characterized as to energy, type, numbers, etc. The results are typically viewed on an oscilloscope and are recorded and analyzed.
One type of detector is a Geiger-Mueller detector ("GM tube") also referred to as a "counter." A GM tube is characteristically operated in a high voltage range from about 500 volts to about 2000 volts, thereby producing a large output signal which is independent of the nature of the initial ionizing event. Because of its potentially extreme sensitivity, a GM tube can be used to detect even very low levels of all types of nuclear particles including beta, gamma and X-rays. It is in relation to the construction of highly reliable, stable and extremely sensitive GM tubes that this invention is specifically concerned.
Sensitive GM tubes are presently used for a variety of purposes in research, medicine and industry. Among the varied uses are: detecting nuclear radiation and recording the type of particles emitted; measuring the change in radioactivity of bombarded materials; measuring and recording cosmic radiation; detecting and tracing radioactive substances in biological systems; using artificially activated substances to follow the progress of chemical and mechanical changes; and locating oil bearing strata in `well logging`. GM tubes are expected to perform reliably even under prolonged harsh conditions incident to their use in such devices as oil level detectors or gauges on aircraft where, during service, the GM tubes are subject to severe vibration and widely fluctuating temperatures, pressures and altitudes. Furthermore, since each tube is used repeatedly, it is important that the operating characteristics of the tube, and particularly its starting voltage, be substantially unaffected by repeated use.
The chamber of a GM tube is filled with a monatomic and/or a diatomic gas which becomes ionized by radiation. Typically a noble gas is used which has desirable ionizing characteristics for the particular type of radiation to be monitored. Such a noble gas commonly used is neon, and to a lesser extent, argon. A quench gas is generally used, in addition to the noble gas in the chamber, to prevent the occurrence of unwanted secondary ionization caused by the release of electrons from the cathode, since a noble gas by itself, does not prevent such occurrence. The quench gas has a lower ionization potential than the noble gas and dissociates to dissipate the excitation energy after pulsing.
Over the years, several quench gases have been used including organic compounds such as ethyl alcohol, ethyl formate and methane, and inorganic halogen gases such as bromine and chlorine. The use of bromine is particularly advantageous because its recombination rate after dissociation is nearly 100%, but because of bromine's relatively high mass, a GM tube containing Br does not have a sufficiently short "dead time" to register count rates in the range from about 1000-1500 counts/sec with accuracy. By "dead time" I refer to the recovery period of the tube after it has registered a discharge, during which period the tube may not be used for making a reading of another discharge. However, the temperature stability and longevity of bromine quenched tubes are outstanding, so that such tubes can be used continuously at temperatures of about 300.degree. C., especially if the cathode is plated with chromium or lined with a tungsten foil liner or sleeve as disclosed in copending patent application Ser. No. 182,375 now U.S. Pat. No. 4,359,661.
It is generally known however, that a halogen quench has two disadvantages. First the negative ion effect is present, as evidenced by a steeper rise to a plateau (in a plot of count rate versus voltage), and a longer rise time of the pulse. Second, because of chemical attack of the cathode, a halogen quench necessitates special procedures, as for example, those described in U.S. Pat. No. 3,892,990 to N. Mitrofanov. For the foregoing reasons, and because bromine has a relatively high electron capture cross section, bromine is not the most desirable quench gas in some applications.
It is with GM tubes having relatively high "useful or relative sensitivity" (or simply "sensitivity") to a low level of ionization, that this invention is particularly concerned. "Sensitivity" is a long-recognized measure of the desirability of a GM tube in situations where the number of events likely to be registered within the tube is small, that is, in the range from about 20 to about 200 counts per second (cts/sec). Sensitivity depends upon the product of (a) the efficiency of production of secondary electrons in the counter by the incident radiation, and (b) the efficiency of the tube counter in discharging once for each such secondary electron formed within its sensitive volume (see Increased Gamma-Ray Sensitivity of Tube Counters and the Measurement of Thorium Content of Ordinary Materials by Robley D. Evans, and Raymond A. Mugele (see Review of Scientific Instruments, 7, 441 et seq (1936). In practice, the measure of sensitivity is the ratio: (number of counts)/(number of gamma quants which reach the surface of the tube), and this ratio is usually represented by (N/n).
Thus, though it is generally accepted that hydrocarbons as a class, almost without exception, have the property of `quenching`, and almost any hydrocarbon can be used as a quenching additive, there is nothing to suggest which hydrocarbon might provide sufficiently good quenching particularly in comparison to a halogen quench, and, therefore, sensitivity and stability to allow the reliable measurement of a count rate of from 20-500 cts/sec. (see "Geiger Counters-Theory and Operation" by Serge Korff, Office of Civil Defense Contract No. GEHC 2068 CO 137, New York University, April 1970). Nor is there any reason to expect that C.sub.2 H.sub.4, alone among the hydrocarbons known to be useful as quenches, would give superior temperature stability and sufficiently short a "dead time" to allow count rates even in the range from about 1000-1500 cts/sec. The design and construction of successful specialty gas tubes is still very much an empirical art.
Since the effect of gas composition on sensitivity and stability of a GM tube does not lend itself to logical deduction, a great number of gas compositions has been tested. For example, ethylene has been used at concentrations greater than 5 percent by volume (% by vol) in conjunction with (a) argon and helium-3 (He.sup.3), and (b) with He.sup.3 alone, as disclosed in "Extraction of Tritium from Helium-3" by Elliott, M. J. W., Rev. Sci. Inst., 31, No. 11, pgs 1218-1222, at 1221 (1960). But the effect of ethylene as a quench gas cannot be deduced from this disclosure, and in fact, at a pressure within the tube of from about 100 mm to about 400 mm mercury (Hg), the concentration of 5% ethylene is too high to be beneficial in the voltage range from about 1000 to about 1500 volts which is required for practical operation of GM tubes.
Apart from the choice of quench gas, and assuming optimum conditions of pressure and voltage are found at which conditions a GM tube has best sensitivity, such sensitivity is known to be enhanced by increasing the effective area of the cathode by employing metal wire screen cathodes or grooved tube cathodes in place of solid smooth cathodes, and by using metal cathodes of high atomic number. Though the increased cathode area due to the use of a screen is primarily responsible for improved sensitivity of a GM tube, there is a well-defined limit as to how much screen can physically be accomodated within a GM tube before there is "arcing over" between the screen and the anode. This is especially critical because the outside diameter of a GM tube is limited by considerations of space, as in the instance where the tube is used for `well logging`; and, of course, the diameter is limited by available voltage for operation of the tube. The optimum amount of plated screen, its surface area and mesh size, are determined by trial and error so as to strike the proper balance between increased surface area which improves sensitivity and the shielding effect which decreases it. The problem still to be solved may be stated with the question: Having struck an appropriate such balance within a GM tube, what further, if anything, can be done to improve its sensitivity?
Thus, metallic screens have been coated with certain heavy metals, that is, metals having high atomic numbers, such as bismuth (Bi) or lead (Pb) and have been used in conjunction with brass cathode tubes. Instead of a plated screen, a single tungsten foil liner or sleeve is known to improve sensitivity, particularly when the foil liner is inserted into a stainless steel cathode of a halogen-quenched GM tube, as disclosed in copending application Ser. No. 182,375, now U.S. Pat. No. 4,359,661 the disclosure of which is incorporated by reference thereto as if fully set forth herein. However, the sensitivity due to such a foil liner in a GM tube cannot be improved upon significantly by adding a second tubular foil liner in electrical contact with the tungsten foil liner based upon an expectation that some improvement in sensitivity would derive from the higher absorption provided by the combined liners. Tests indicate that a 2 mil tungsten foil cathode liner actually reduces sensitivity for low energy gamma rays below 0.1 Mev, and there is no significant improvement in sensitivity irrespective of the type of heavy metal from which the foil liner is fabricated, or if an additional foil liner of any heavy metal is added. It is now clear from experimental evidence that a 1 mil thickness of foil liner is the maximum thickness which is preferably used for energy levels in the range from about 120 keV to about 1250 keV, since a greater thickness than 1 mil, even if such greater thickness is derived by plating a deposit of heavy metal on the interior surface of the outer cathode, serves only to reduce sensitivity in the stated energy range. In certain special circumstances where very high energy levels are expected, the thickness of the foil liner may be up to about 2 mil thick, but energy levels in excess of 1 Mev are of little concern in GM tubes of this invention. Since a foil liner and a plated deposit of heavy metal on the inner surface of a cathode, has each been discovered to provide an equivalent function of improvement in sensitivity, though effective thickness is increased in a different manner, they are each referred to in this specification as a "sleeve".
Though a single cathode liner, whether screen or sleeve, has each been used in the prior art, there is nothing to suggest that a combination of screen and sleeve cathodes, such as a dual-sleeve cathode, might have any desirable effect on sensitivity or stability, much less that such desirable properties as each may have been evaluated individually, might actually be improved. Considering that any evaluation of the probable effects of "ganging up" liners is reasonably to be made relative to a particular range of energy levels in which the GM tubes are to be operated, it appeared that increasing the effective thickness of liners for tubes to be operated in the range from 122 keV to about 1250 keV, was contraindicated.
It is known that as the thickness of material exposed to the gamma radiation is increased, scattering will obscure the initial direction of emission of electrons, and in a thick foil of high Z material, effective emission will be isotropic for all processes; also, that whatever the material of the foil, the maximum electron production will occur for a foil of thickness equal to the range of the photooelectron. (see "Nuclear Radiation Detectors", by Jack Sharpe, pg 91 et seq., Methuen & Co. Ltd., London (1964). The identity of the particular metals through which the photoelectron travels is surprisingly unimportant, the range in lead being only about 25 percent less than that in aluminum, stated in mg/cm.sup.2. For gamma rays in the range from about 122 keV to about 1250 keV, the range of the photoelectron in a heavy metal is calculated to be less than 20 mg/cm.sup.2, recognizing this is inapplicable in the Compton range. Therefore, it would be expected that any increase in effective thickness of material greater than 20 mg/cm.sup.2 would decrease sensitivity, particularly as the coating of heavy metal is on wire which itself is at least 10 mil thick, or it would be difficult to weave the wire into a screen.
Since, from a theoretical point of view, effective thickness of the liner directly confronts gamma radiation to which the GM tube is exposed, it appeared equally improbable that ganging up a sleeve with a screen which already has a heavy metal coating of at least 15 mg/cm.sup.2 of screen surface (equivalent to 0.285 mil thickness of metal), to provide a greater net effective thickness than 0.375 mil, would provide an increase in either sensitivity of the tube, or its stability. A thickness of 0.375 mil is produced by plating about 18 gm/cm.sup.2. It would therefor be expected that a cathode plated with in excess of 18 gm/cm.sup.2 of a heavy metal would exhibit decreased sensitivity. It does not. Moreover, by inserting a screen, plated with a heavy metal, into the plated cathode tube, or into the foil liner inside the cathode tube, one would expect a further decrease in sensitivity because of the increase in net effective thickness of heavy metal. It does not.
In view of the foregoing it was especially unexpected that the addition of a screen liner, in addition to a sleeve, (whether the foil liner or the plated deposit), should be a desirable combination for measuring gammas in the range from about 122 keV to about 0.67 Mev. In the GM tubes of this invention, quite unexpectedly, such a combination is.